This invention relates to electron guns, and especially to electron guns for use in television picture tubes. The invention is particularly directed to electron lenses for such guns, and more particularly to long focal length lenses (extended lenses) of the resistive type.
One of the characteristics which measure the quality of an electron lens is its degree of freedom from spherical aberration. In the presence of spherical aberration, all electrons emanating from an object point do not, after focusing, recombine at a common image point.
Spherical aberration can be reduced by using an electron lens having a long focal length. Long focal length lenses are characterized by weak focus fields having slightly, rather than sharply, curved field lines which are established over a longer path of beam travel. Accordingly, such lenses are also referred to as extended lenses because the lensing action is "stretched out" or "extended" over a longer distance. The weak fields of such lenses subject an electron beam passing therethrough to a gentle, gradual bending of its rays, thereby avoiding severe spherical aberration.
It is well known that the focal length of a lens can be lengthened by increasing the size of the lens aperture and/or the gap between two electrodes on the lens. However, increasing the diameter of the lens conflicts with the desire to dispose the electron gun in a small neck of a cathode ray tube in order to minimize required deflection power. On the other hand, if the gap between the electrodes is made too large, other electric fields external to the lens penetrate the gap and distort the focus field.
The prior art has disclosed various extended lens structures designed to achieve longer focal length without the attendant disadvantages described above. One type of extended lens is exemplified by the following three representative patents:
______________________________________ U.S. Pat. No. Inventor Issue Date ______________________________________ 3,863,091 Hurakawa et al January 28, 1975 3,895,253 Scwartz et al July 15, 1976 3,995,194 Blacker et al November 30, 1976 ______________________________________
In lenses as disclosed in these three patents, the lensing action is extended over a longer distance of beam travel by the inclusion of additional electrodes to which different voltages are applied. In such guns, it has been the practice to provide a separate lead-in for each of the different voltages. The increased lead-in requirement therefore becomes a significant disadvantage to this type of lens.
Another type of extended lens is exemplified by the following two representative patents:
______________________________________ U.S. Pat. No. Inventor Issue Date ______________________________________ 2,143,390 Schroter January 10, 1939 2,291,462 Gardner July 28, 1952 ______________________________________
In the electron lenses of FIG. 3 of Schroter and of the Gardner patent, a tubular member of resistive material is provided, and a continuous voltage gradient is established along the member by the application of two different potentials to its two ends. This type of lens suffers from the disadvantage of an unavailability of suitable materials from which to make the resistive member and/or unavailable state of art technology for fabricating the member.
A third class of extended lens is exemplified by FIG. 1 of the Schroter patent and by U.S. Pat. No. 3,932,786 issued to F. J. Campbell, on Jan. 13, 1976. In this type of lens, a plurality of metal electrode plates are arranged in serial fashion and a voltage gradient is established along the lens by applying different voltages to the different plates. In order to minimize the number of required lead-ins, a resistive element is provided within the vacuum envelope of the electron tube itself. Although Schroter shows this resistor only schematically, Campbell discloses a practical embodiment of a bleeder resistor disposed on an insulator element of the electron gun structure. Nevertheless, in practice the Campbell structure has proved to have attendant problems of stray emission because of the many connectors required to make contact between the series of apertured electrodes and the bleeder resistor.
Of the extended lenses described above, the Schroter, Gardner, and Campbell structures can be classified as resistive lenses. This is because these three structures incorporate a resistive element within the vacuum enclosure of the electron tube itself, so that only two lead-ins are required to provide a voltage gradient along the length of the lens.